Vapor generator

ABSTRACT

A forced flow vapor generator wherein the upright boundary walls of the furnace are subdivided into a plurality of separate continuous upflow fluid heating passes, with the parallel flow tubes of one of the fluid heating passes being interlaced and coextensive with the parallel flow tubes of another of the fluid heating passes, with provisions for interconnecting the tubes of the fluid heating passes to provide serial flow of fluid successively through the respective fluid heating passes and for mixing the fluids to equalize fluid enthalpies as they flow from one furnace fluid heating pass to another, and with the number and the total internal cross-sectional area of the tubes of one fluid heating pass being greater than the number and total internal cross-sectional area of the tubes in the other fluid heating pass to provide a fluid mass flow in the latter greater than the fluid mass flow in the former.

United States Patent [191 Frendberg et a1.

[ VAPOR GENERATOR [75] Inventors: Arthur M. Frendberg, Akron, Ohio;

Walter P. Gorzegno, Florham Park, NJ.

[73] Assignee: The Babcock & Wilcox Company, New York, N.Y.

[22] Filed: July 9, 1965 [21] App]. No.: 470,819

[52] US. Cl. 122/510, 122/406 [51] Int. Cl. F22b 37/24 [58] Field of Search..... 122/406, 406 S, 235, 235 A [111 3,834,358 [451 Sept. 10,1974

Primary Examiner-Kenneth WiS prag ue Assistant Examiner-Ronald C. Capossela Attorney, Agent, or FirmJoseph M. Maguire, Esq.

[5 7] ABSTRACT A forced flow vapor generator wherein the upright boundary walls of the furnace are subdivided into a plurality of separate continuous upflow fluid heating passes, with the parallel flow tubes of one of the fluid heating passes being interlaced and coextensive with the parallel flow tubes of another of the fluid heating passes, with provisions for interconnecting the tubes of the fluid heating passes to provide serial flow of fluid successively through the respective fluid heating passes and for mixing the fluids to equalize fluid enthalpies as they flow from one furnace fluid heating pass to another, and with the number and the total internal cross-sectional area of the tubes of one fluid heating pass being greater than the number and total internal cross-sectional area of the tubes in the other fluid heating pass to provide a fluid mass flow in the latter greater than the fluid mass flow in the former.

11 Claims, 6 Drawing Figures Pmmansmm 3,834,358

SHEEI 1 8- 3 'INVENTORS Arthur M. Frendberg ATTORNEY Walrer P Gorzegno PATENIED SEP 1 01974 MEI2U3 VAPOR GENERATOR The present invention relates in general to the construction and operation of a forced flow fluid heating unit and more particularly to improvements in the construction and arrangement of fluid heating circuits especially adapted for use in a forced circulation oncethrough steam generating and superheating unit.

The construction of forced circulation once-through steam generators requires the use of a large number of parallel flow circuits connected between inlet and outlet headers. One of the fundamental problems involved with such a steam generator is the control of the flow through the various parallel flow circuits in order that the flow in each circuit will be stable and the enthalpy of the fluid discharged from any individual circuit will be close to the average of that from all circuits, in which case the circuit will be in a balanced flowcondition. Unbalanced flow may be caused by unequal heat absorption in parallel flow circuits due to an unsymmetrical arrangement of heating surface, slag accumulation, or part-load operation with burners out of service; or may be due to unequal resistances caused by different lengths of circuits. When steam or water, or mixtures thereof, is heated in parallel flow paths provided by the furnace wall tubes or tubular panels disposed in the furnace, unbalanced heat and/or fluid distribution may lead to excessive localized tube metal temperature and/or to excessive temperature differentials between adjacent furnace wall tubes and, thereby, to undue thermal stresses in the furnace wall-forming components.

Many variations of furnace wall fluid heating circuitry have been applied to vapor generators of the character described. Mostof these have one or more shortcomings including excessive thermal stresses, uneven thermal expansion, and/or lack of sufficient stability against transient conditions of heat absorption inherent in the operation of a vapor generator of the character described. For example, fluid heating circuitry of the meandering type or of the type including heated upflow and downflow tubes may present stability difficulties, while furnace boundary walls of the type having serially connected side-by-side tube panels welded together may give rise to thermal expansion movement and stresses requiring special solutions.

The invention of US. patent application Ser. No. 447,699 generally meets the foregoing problems by subdividing the furnace into a plurality of serially connected fluid heating passes with parallel flow tubes of the first of the heating passes being substantially the same in number as and interlaced and coextensive with parallel flow tubes of the second of the fluid heating passes. This flow system has been found most effective when embodied in units of high or intermediate steaming capacities. While such a system may be applied to units of relatively small capacity, certain economic and functional difficulties are encountered.

One of the criteria for sizing a furnace is the limit for heat input per square foot of gas flow area. This limit determines the plan cross section of a furnace. For a given fuel this plan cross section is proportional to the full load steam output of the unit. With the usual rectangular cross section, the furnace periphery increases as the square root of the full load steam output. The quantity of full load working fluid flow per foot of furnace wall periphery decreases with the unit size and for small units with serially connected interlaced fluid heating passes each having substantially the same number of parallel flow tubes, the full load fluid flow per foot of furnace periphery may become insufficient to maintain tube wall temperatures within acceptable limits. Although under these circumstances the internal area of tubes of the fluid heating passes may be reduced to give adequate mass flow, the tube diameter becomes so small as to render this measure economically impractical in respect of the cost of manufacturing to the close tolerances needed to minimize maldistribution of fluid and functionally impractical in respect of the relatively high susceptibility to plugging of a tube of the size required. The problem of providing for adequate fluid mass flow in the tubes of the fluid heating passes of once-through boilers of relatively small capacity is solved in accordance with the present invention by increasing the ratio of the number of tubes in the first furnace fluid heating pass to the number of tubes in the succeeding fluid heating pass of the furnace. In accordance with the invention, in a unit of the character described the upright boundary walls of the furnace are subdivided into a plurality of separate continuous upflow fluid heating passes, with the parallel flow tubes of one of the fluid heating passes being interlaced and coextensive with the parallel flow tubes of another of the fluid heating passes, with provisions for interconnecting the tubes of the fluid heating passes to provide serial flow of fluid successively through the respective fluid heating passes and for mixing the fluids to equalize fluid enthalpy as they flow from one furnace fluid heating pass to another, and with the number and the total internal cross-sectional area of the tubes of one fluid heating pass being greater than the number and total internal cross-sectional area of the tubes in the other fluid heating pass to provide a fluid mass flow in the latter greater than the fluid mass flow in the former.-

The various features of novelty which characterize our invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which we have illustrated and described a preferred embodiment of the invention.

Of the drawings:

FIG. 1 is a sectional elevation of a once-through forced flow steam generator embodying the invention;

FIG. 2 is a partial sectional view taken along the line 22 of FIG. 1;

FIG. 3 is an enlarged view of a portion of the front wall fluid collection, mixing and distribution system shown in FIG. 1;

FIG. 4 is a view taken along the line 4-4 of FIG. 3;

FIG. 5 is an enlarged view of a portion of the rear wall fluid collection, mixing and distribution system shown in FIG. 1; and

FIG. 6 is a view taken along the line 6-6 of FIG. 5.

In the drawings the invention has been illustrated as embodied in a top-supported forced flow once-through steam generator intended for central station use. The particular unit illustrated is designed to produce a maximum continuous steam output of 2,450,000 lbs/hour at a pressure of 3625 psig and a total temperature of l000F at the superheater outlet, based on feedwater being supplied at a temperature of 540 1 with provisions for reheating the steam.

The main portions of the unit illustrated include an upright furnace chamber 10 of substantially rectangular horizontal cross section defined by front wall 11, rear wall 13, side walls 14, a roof 16 and a floor 17 and having a gas outlet 18 at its upper end opening to a horizontally extending gas pass 19 of rectangular vertical cross section formed by a floor 21 and extensions of the furnace roof l6 and side walls 14. Gas pass 19 communicates at its rear end with the upper end of an upright gas passage 22 of rectangular horizontal cross section formed by a front wall 23, a rear wall 24, side walls 26 and an extension of the roof of the gas pass 19.

The fuel firing section comprises independently operable horizontally extending cyclone type furnaces 27 of relatively small volume and boundary wall area disposed on oppositewalls 11 and 13 at the lower portion of the furnace chamber 10. Each cyclone furnace is arranged to burn solid fuel at high rates of heat release and separately discharge high temperature gaseous products of combustion and separated ash residue as a molten slag into the lower portion of the chamber 10. Floor 17 is formed with suitable openings, not shown, for the discharge of molten slag to a slag tank, not shown.

Gas pass 19 is occupied by a secondary superheater 28, a high pressure reheater section 29 and a part of a low pressure reheater 31 arranged in series with respect its boundary walls lined or formed by tube panels constructed and arranged in a manner similar to that described in US. Pat. No. 3,081,748. Thev high pressure fluid from the downcomer 35 flows in parallel to cyclone furnace supply headers 40 by way of supply tubes 45, each one of the parallel flow streams passing through the circumferential wall tubes of the corresponding cyclone furnace to a discharge header 50. Streams of fluid discharging from headers 50 are collected in a conduit 36 for flow to the furnace boundary wall fluid heating circuitry, which will be hereinafter described. From the furnace boundary wall fluid heatto gas flow; while gas pass 22 is occupied in the direction of gas flow by the remainder of the low pressure reheater 31, a high pressure reheater section 32, a primary superheater 33 and an economizer 34.

In the normal operation of the fluid heating unit, combustion air and a relatively .coarse crushed fuel are supplied to the cyclone furnaces from independently controllable sources and the fuel is burned in the cyclone furnaces at high rates of heat release sufficient to maintain a normal mean temperature therein above the fuel-ash fusion temperature. Ash separates as a molten slag which flows into the lower portion of the chamber 10 and is discharged to the slag tank, while gases with a relatively small amount of slag particles in suspension discharge into the lower portion of the chamber 10. The heating gases then flow upwardly through chamber 10 to the inlet of gas pass 19, then pass successively over and between the tubes of secondary superheater 28, reheater 29, reheater 31, reheater 32,.Primary superheater 33 and economizer 24, and then discharge to another heat trap, not shown, before flowing to the stack. It will be understood that in accordance with well-known practice, each of the superheaters and re heaters extends across the full width of its corresponding gas pass and is formed for serial flow of steam by multiple looped tubes.

The vapor generating unit is top-supported by structural steel members including upright members 85 and cross beams 90 from which hangers 95, of which only a few are illustrated,-support all walls.

Feedwater at high pressure is supplied by a feed pump, not shown, to economizer inlet header 25, then passes through economizer 34 to outlet header 30 from which it flows through a downcomer to the cyclone furnace fluid heating circuits. Each cyclone furnace has ing circuitry the fluid passes to a common mixing header 66 which is arranged to distribute the fluid to tubes 67 forming the roof of furnace 10 and gas passes 19 and 22 and having their discharge ends connected to a header 68. From header 68 the fluid flows through a conduit 69 for distribution to boundary wall tubes of gas passes 19 and 22.

Each of the upright boundary walls of gas passes 19 and 22 includes upright parallel tubes, front wall 23 having tubes71 extending between inlet and outlet headers 72 and 73, rear wall 24 having tubes 74 extending'between inlet and outlet headers 76 and 77, each side wall 26 having tubes 78 extending between inlet and outlet headers 79 and 81, and each side wall of gas pass 19 having tubes 82 extending between inlet and outlet headers 83 and 84. Floor 21 is lined by a row of tubes 86 having their inlet ends connected to a header 87 and their outlet ends to header 73, with header 73 being connected for flow of fluid to a header 88 by a row of screen tubes 89. Headers 72, 76, 79, 83, and 87 are connected for parallel supply of fluid from conduit 69, while headers 77, 81, 84 and 88 are arranged for discharge to a common collection header 91 from which fluid passes to the primary superheater 33 by way of a conduit 92. From primary superheater 33 the partly superheated vapor passes to secondary superheater 28 within which the vapor receives its final superheating before passing to a high pressure turbine, not shown. Partially expanded steam from the high pressure turbinesuccessively passes through reheater sections 32 and 29 to and through an intermediate pressure turbine, not shown, then flows through reheater 31 to a low pressure turbine, not shown, wherein final expansion takes place.

In accordance with the invention, each of the upright boundary walls of furnace 10 is formed by upwardly extending parallel tubes arranged to provide three upflow fluid heating passes and having their intertube spaces closed by metallic webs welded to adjacent tubes to provide a gas-tight construction. Special provisions are made for mixing the heat absorbing medium as it flows from one pass to another, the mixing system between each of the furnace fluid heating passes being used to keep the wall tube temperature differences to a minimum. With differences in furnace cleanliness and variation in flow quantities in the parallel flow tubes of a fluid heating pass it is possible to develop a temperature difference between adjacent tubes of a magnitude placinghigh stresses on the tubes and the metallic webs therebetween. By limiting the magnitude of the B.t.u.

pickup in any furnace fluid heating pass the degree of the temperature unbalance is also limited. Accordingly, the furnace boundary wall fluid heating surface is so proportioned and arranged that the temperature of the fluid in any tube at any furnace level differs no more than 100F from the average fluid temperature of all furnace wall tubes at that level; that the maximum temperature differential between adjacent tubes is below a predetermined critical limit; that fluid flow unbalances are minimized; and that the tubes of each fluid heating pass are sufficient in number and in inside diameter along their lengths to provide adequate circulation velocities. Further, all heated tubes of the furnace boundary walls are arranged for upflow of fluid, for the stability of fluid heating passes having their tubes so arranged is markedly improved compared to circuitry having both heated downflow as well as upflow tubes. The flow unbalances for the same average and upset heat absorption conditions are considerably less with all heated upflow tubes in a fluid heating pass than with heated downflow and upflow tubes in such a pass. Thus front wall 11 comprises initial upflow tubes 37A, second upflow tubes 37B disposed in the spaces between initial upflow tubes 37A and third upflow tubes 37C. Rear wall 13 includes initial upflow tubes 38A, second upflow tubes 38B situated in the spaces between tubes 38A, and third upflow tubes 38C. Tubes 38C provide support for tubes 38A and 38B, extend across the inlet to gas pass 19 to form a screen, and have their lower ends disposed in spaces between floor tubes 86. Each side wall 14 has initial upflow tubes 39A, second upflow tubes 39B located in the spaces between tubes 39A, and third upflow tubes 39C. Floor 17 is lined by a row of tubes 42 extending between an inlet header 43 and an outlet header 44 with header 43 being arranged for supply of fluid from conduit 36 and header 44 being connected by conduits 46 for parallel discharge of fluid to headers 47A, 47B and 47C disposed outside of the lower end of furnace l0 and adapted to supply fluid to initial upflow tubes 37A, 38A and39A, respectively.

Initial upflow tubes 37A, 38A and 39A of the front, rear and side walls of furnace have their outlet ends connected to a ring shaped header 49 extending about and outside of furnace 10 at a level somewhat below the lower end of gas outlet 18. Fluid passing through initial upflow tubes 37A, 38A and 39A is collected in header 49 and then passed through a conduit 51 to a ring shaped header 52 disposed about and outside of furnace 10 at around the level of floor 17 and arranged to supply fluid to second upflow tubes 37B, 38B and 39B. The second upflow tubes of the front, rear and side walls of furnace 10 extend from the floor 17 to a level intermediate the lower end of gas outlet 18 and the header 49 and have their upper ends connected to horizontal headers 53, 54 and 56, respectively located superjacent header 49.

Third pass upflow tubes 37C, 38C and 39C extend from a level just below the lower end of gas outlet 18 to the top of the furnace, tubes 37C extending between horizontal inlet and outlet headers 57 and 58, tubes 38C between horizontal inlet and outlet headers 59 and 61, and tubes 39C between horizontal inlet and outlet headers 62 and 63, with headers 57, 59 and' 62 being respectively connected for supply of fluid from headers 53, 54 and 56 by fluid mixing conduits 55, 65, and 75. Headers 57, 59 and 62 arelocated subjacent and extend parallel to headers 53, 54 and 56, respectively,

and are situated superjacent and extend parallel to the portion of header 49 of the corresponding wall.

From the above description it is evident that tubes 37A, 38A and 39A constitute the first fluid heating pass of the furnace, tubes 37B, 38B and 39B the second fluid heating pass, and tubes 37C, 38C and 39C the third fluid heating pass, with the ratio of the number of first fluid heating pass tubes to second fluid heating pass tubes being 3 to 2 for reasons hereinafter indicated. The tubes of the first and second fluid heating passes of each upright wall of the furnace are coplanar along almost their entire extent, coextend from the floor 17 to a level just below the lower end of gas outlet 18, and have their intertube spaces closed by metallic webs 100 weld-united to the tubes along substantially their entire lengths. Tubes of the third fluid heating pass extend from a level just below the lower end of gas outlet 18 to a roof l6 and, with the exception of tubes 38C, have their intertube spaces closed by metallic webs 101 weld-united to the tubes along substantially their entire lengths. Tubes of the third fluid heating pass of each upright wall of the furnace are coplanar along almost their entire extent and are coplanar with the tubes of the first and second fluid heating passes of the corresponding wall.

Since the construction and arrangement of the fluid collection, mixing and distribution systems and their associated tubes are substantially the same in the side and front walls, of these the construction and arrangement of only the front wall system and itsassociated fluid heating passes will be described. The discharge portions of tubes 37A are bent outwardly from the plane of the wall at about the level of header 57 and then extend downwardly and outwardly for radial connection to ring header 49 wherein the fluids discharging from the first fluid heating pass are collected. From header 49 the fluids pass through conduit 51 wherein the fluids are mixed to neutralize the differences in amount of heat picked up in the first fluid heating pass. The fluids so mixed discharge to header 52 which provides uniform distribution of the fluids to parallel flow tubes 37B of the second fluid heating pass. Discharge portions of tubes 37B are bent outwardly from the plane of the wall at about the level of header 53 for radial connection to header 53 where the fluids discharging from the front wall tubes of the second fluid heating pass are collected and then passed through conduit 55 wherein the fluids are mixed to neutralize the differences in amount of heat picked up in the second fluid heating pass. From conduit 55 the mixed fluids discharge to header 57 for uniform distribution to parallel flow tubes 37C of the front wall third fluid heating pass. lnlet portions of some of the tubes 37C, in a number corresponding to the number of tubes 37A, extend radially from header 57 to enter front wall 11 at a position above and in nearly contacting relation with the discharge portions of tubes 37A and then extend upwardly in the plane of the wall between tubes 37B and in alignment with wall tubes 37A. lnlet portions of the remaining tubes 37C, in a number corresponding to the number of tubes 37B, extend generally upwardly from header 57 to enter front wall 11 at a location above and in nearly contacting relation with the discharge portions of tubes 37B, and then extend upwardly in the plane of the wall in alignment with wall tubes 37B. Webs 100 close the spaces between tubes 37A and 37B up to about the level of header 57, while webs 101 close the spaces between tubes 37C down to about the level of header 53. The intertube spaces intermediate headers 57 and 53 are closed by metallic webs 102 except at the point where tubes 37A, 37B and 37C bend out of the plane of the wall. Wall seals are provided at these points by H-shaped plates 103 suitably welded to the adjacent tube portions.

In the rear wall, discharge portions of tubes 38A are bent outwardly from the plane of the wall at a level somewhat above header 54 and then extend generally downwardly and outwardly for radial connection to ring header 49 wherein the fluids discharging from the first fluid heating pass of the rear wall are collected, From header 49 the fluids pass through conduit 51 to header 52 which provides uniform distribution of fluids to parallel flow tubes 38B of the second fluid heating pass of the rear wall. Discharge portions of tubes 38B are radially connected to header 54 and bent outwardly from the plane of the wall at the same level as the discharge portions of tubes 38A are so bent. Fluids discharging from the rear wall tubes of the second fluid heating pass are collected in header 54 and then pass through conduit 65 to header 59 for uniform distribution to parallel flow tubes 38C of the rear wall third fluid heating pass. Inlet portions of tubes 38C extend radially from header 59 to enter rear wall 11 at the level of header 59 and then extend upwardly in the plane of the wall, with certain of the first and second fluid heating pass tubes being suitably bent out of the plane of the wall to permit such routing of tubes 38C.

Webs 100A close the spaces between tubes 38A and 38B up to about the level of header 59, while webs 101A close the spaces between floor tubes 86 and tubes 38C down to about the level at which tubes 38A and 38B are bent out of the plane of the rear wall. The

intertube spaces intermediate header 59 and the level at which tubes 38A and 38B bend out of the plane of the wall are closed by metallic webs 102A except at the points where tubes 38A and 38B bend out of the plane of the wall. Wall seals are provided at these points by l-l-shaped plates 103A suitably welded to the adjacent tube portions.

Thus at the locations provided for mixing of the fluids as they flow from one furnace fluid heating pass to another, the tubes of the second fluid heating pass are interlaced with those of the first and third fluid heating passes and cooperate therewith and with the webs and plates therebetween to provide a gas-tight structure.

in operation high pressure fluid discharges from conduit 36 to header 43; then passes through floor tubes 42; then flows upwardly in parallel through the radiant heat absorbing initial upflow tubes 37A, 38A and 39A of the front, rear and side walls of the furnace to collecting header 49; then passes through conduit 51 to header 52 while being mixed; then flows in parallel upflow through second upflow tubes 37B, 38B and 398 to collecting headers 53, 54 and 56, respectively, the fluids from these headers respectively passing to fluid dis tribution headers 57, 59'and 62 by way of fluid mixing conduits 55, 65 and 75; then flows in'parallel through third upflow tubes 37C, 38C and 39C to headers 58, 61 and .63, respectively; and then passes to header 66 for distribution to furnace roof tubes 67.

For a given location in the furnace the required mass flow in the tubes depends on the fluid enthalpy. At lower enthalpies the required mass flow for proper cooling of the tube metal will generally be lower. The enthalpy of the fluid in the first fluid heating pass of furnace is lowerv than in the second fluid heating pass. in accordance with the'invention, to provide adequate fluid-mass flow for maintaining tube wall temperatures within acceptable limits in units of relatively small full load steam output, the number and total internal crosssectional area of the tubes of the first fluid heating pass should be substantially greater than the number and total internal cross-sectional area of the tubes of the second fluid heating pass to provide a fluid mass flow in the second fluid heating pass greater than the fluid mass flow in the first fluid heating pass. The ratio of the number of tubes in the first fluid heating pass to the number of tubes of the second fluid heating pass can vary depending on the mass flow requirements. It has been determined that for many units the best combination is achieved with three first pass tubes for every two second pass tubes. As shown in FIG. 2, the first and second pass tubes are interlaced around the periphery of the furnace in a pattern l2121---- l212l---- The three-to-two split permits an increase in the mass flow in the second fluid heating pass by 25 percent over what it would be in the second fluid heating pass with a one-to-one split using the same total number of tubes. This means, of course, that the mass flow in the first fluid heating pass of the three-to-two split is 16.7 percent less than in the one-to-one arrangement. One of the prime benefits of the three-to-two split is reduction of the metal temperature differentials between the interlaced tubes of the first and second fluid heating passes. By reducing the mass flow in the first fluid heating pass, which has a cooler fluid, the tube metal temperature is raised. The higher mass flow in the second fluid heating pass, which contains the hotter fluid, tends to maintain the tube metal temperatures relatively low. By adjusting the mass flows of the respective fluid heating passes by increasing the ratio of tubes in the first pass to the second pass, it is possible to bring the metal temperatures of the two interlaced fluid heating passes very close to each other so that little or no temperature differences will exist between them. Thus stresses due to temperature differences are minimized.

While in accordance with the provisions of the statutes, we have illustrated and described herein the best form and mode of operation of the invention now known to us, those skilled in the art will understand that changes may be-made in the form of the apparatus disclosed without departing from the spirit of the invention covered by our claims, and that certain features of the invention may sometimes be 'used to advantage without a corresponding use of other features.

We claim:

1. In a forced circulation fluid heating unit, upright walls forming a furnace for flow of heating gases, means supplying high temperature heating gases to said furnace, one of said walls having a first fluid heating pass comprising a multiplicity of upflow tubes arranged in spaced relation and for parallel flow of fluid therethrough, a second fluid heating pass comprising a mul-. tiplicity of upflow tubes interlaced with upflow tubes of the first fluid heating pass and arranged for parallel flow of fluid therethrough, said first and second fluid heating passes being constructed and arranged to provide flow of fluid in the furnace only in an upward direction, means supplying a vaporizable fluid to the fluid heating passes, means interconnecting the fluid heating passes to provide serial flow of fluid therethrough, the

number and the total internal cross-sectional area of the tubes of one of the fluid heating passes being substantially greater than the numberand total, internal cross-sectional area of the tubes of the other fluid heating pass to provide a fluid mass flow in the latter greater than the fluid mass flow in the former, and means rigidly uniting tubes of the first and second fluid heating passes.

2. In a forced circulation fluid heating unit, upright walls forming a furnace for flow of heating gases, means supplying high temperature heating gases to said furnace, one of said walls having a first fluid heating pass comprising a multiplicity of upflow tubes arranged in spaced relation and for parallel flow of fluid therethrough, a second fluid heating pass of substantially the same length as the first fluid heating pass comprising a multiplicity of upflow tubes interlaced and coextensive with upflow tubes of the first fluid heating pass and arranged for parallel flow of fluid therethrough, said first and second fluid heating passes being constructed and arranged to provide flow of fluid in the furnace only in an upward direction, means for passing a vaporizable fluid to tubes of the first fluid heating pass, means interconnecting said fluid heating passes to provide serial upflow of fluid successively through tubes of the first fluid heating pass and tubes of the second fluid heating pass, the number and the total internal cross-sectional area of the tubes of one of the fluid heating passes being substantially greater than the number and total internal cross-sectional area of the tubes in the other fluid heating pass to provide a fluid mass flow in the latter greater than the fluid mass flow in the former, and metallic web means rigidly uniting tubes of the first and second fluid heating passes.

3. In a forced circulation fluid heating unit, upright walls forming a furnace for flow of heating gases, means supplying high temperature heating gases to said furnace, one of said walls having a first fluid heating pass comprising a multiplicity of upflow tubes arranged in spaced relation and for parallel flow of fluid therethrough, a second fluid heating pass comprising a multiplicity of upflow tubes interlaced with upflow tubes of the first fluid heating pass and arranged for parallel flow of fluid therethrough, said first and second fluid heating passes being constructed and arranged to provide flow of fluid in the furnace only in an upward direction, means for passing a vaporizable fluid to tubes of the first fluid heating pass, means interconnecting said fluid heating passes to provide serial upflow of fluid successively through tubes of the first fluid heating pass and tubes of the second fluid heating pass, the number and the total internal cross-sectional area of the tubes in the first fluid heating pass being substantially greater than the number and total internal crosssectional area of the tubes in the second fluid heating pass to provide a fluid mass flow in the second fluid heating pass greater than the fluid mass flow in the first fluid heating pass, and means rigidly uniting tubes of the first and second fluid heating passes.

4. In a forced circulation fluid heating unit, upright walls forming a furnace for flow of heating gases, means supplying high temperature heating gases to said furnace,one of said walls having a first fluid heating pass comprising a multiplicity of upflow tubes arranged in spaced relation and for parallel flow of fluid therethrough, a second fluid heating pass ofsubstantially the same length as the first fluid heating pass comprising a multiplicity of upflow tubes interlaced and coextensive with upflow tubes of the first fluid heating pass and arranged for parallel flow of fluid therethrough, said first and second fluid heating passes being constructed and arranged to provide flow of fluid in the furnace only in an upward direction, means for passing a vaporizable fluid to tubes of the first fluid heating pass, means interconnecting said fluid heating passes to provide serial upflow of fluid successively through tubes of the first fluid heating pass and tubes of the second fluid heating pass, the number and the total internal cross-sectional area of the tubes in the first fluid heating pass being substantially greater than the number and total internal cross-sectional area of the tubes in the second fluid heating to provide a fluid mass flow in the second fluid heating pass greater than the fluid mas's flow in the first fluid heating pass, and metallic web means rigidly uniting tubes of the first and second fluid heating passes along substantially their entire lengths.

5. In a forced circulation fluid heating unit, upright walls forming a furnace for flow of heating gases, means supplying high temperature heating gases to said furnace, one of said walls having a first fluid heating pass comprising a multiplicity of upflow tubes arranged in spaced relation and for parallel flow of fluid therethrough, a second fluid heating pass comprising a multiplicity of upflow tubes interlaced with upflow tubes of the first fluid heating pass and arranged for parallel flow of fluid therethrough, a third fluid heating pass comprising a multiplicity of upflow tubes above the tubes of the first and second fluid heating passes, means for passing a vaporizable fluid to tubes of the first fluid heating pass, means interconnecting said fluid heating passes to provide serial upflow of fluid successively through tubes of the first fluid heating pass, tubes of the second fluid heating pass and tubes of the third fluid heating pass, the number and the total internal crosssectional area of the tubes in the first fluid heating pass being substantially greater than the number and total internal cross-sectional area of the tubes in the second fluid heating pass to provide a fluid mass flow in the second fluid heating pass greater than the fluid mass flow in the first fluid heating pass, means rigidly uniting tubes of the first and second fluid heating passes, and means rigidly uniting tubes of the third fluid heating pass.

6. In a forced circulation fluid heating unit, upright walls forming a furnace for flow of heating gases, means supplying high temperature heating gases to said furnace, one of said walls having a first fluid heating pass comprising a multiplicity of upflow tubes arranged in spaced relation and for parallel flow of fluid therethrough, a second fluid heating pass of substantially the same length as the first fluid heating pass comprising a multiplicity of upflow tubes interlaced and coextensive with upflow tubes of the first fluid heating pass and arranged for parallel flow of fluid therethrough, a third fluid heating pass comprising a multiplicity of upflow tubes above the tubes of the first and second fluid heating passes, means for passing a vaporizable fluid to tubes of the first fluid heating pass, means interconnecting said fluid heating passes to provide serial upflow of fluid successively through tubes of the first fluid heating pass, tubes of the second fluid heating pass and tubes of the third fluid heating pass, the number and the total internal cross-sectional area of the tubes in the first fluid heating pass being substantially greater than the number and total internal cross-sectional area of the tubes in the second fluid heating pass to provide a fluid mass flow in the second fluid heating pass greater than the fluid mass flow in the'first fluid heating pass, r metallic web means rigidly unitingtubes of the first and second fluid heating passes along substantially their entire lengths, and metallic web means rigidly uniting tubes of the third fluid heating pass along substantially their entire lengths.

7. In a forced circulation fluid heating unit, upright walls forming a furnace for flow of heating gases, means supplying high temperature heating gases to said furnace, one of said walls having a first fluid heating pass comprising a multiplicity of upflow tubes arranged in spaced relation and for parallel flow of fluid therethrough, a second fluid heating pass comprising a multiplicity of upflow tubes interlaced with upflow tubes of the first fluid heating pass and arranged for parallel flow of fluid therethrough, a third fluid heating pass comprising a multiplicity of upflow tubes above the tubes of the first and second fluid heating passes, means for passing a vaporizable fluid to tubes of the first fluid heating pass, means interconnecting said fluid heating passes to provide serial upflow of fluid -successively through tubes of the first fluid heating pass, tubes of the second fluid heating pass and tubes of the third fluid heating pass, tubes of the third fluid heating pass having lower portions interlaced with upper portions of tubes of the second fluid heating pass and aligned with tubes of the first fluid heating pass, the number and the total internal cross-sectional area of the tubes in the first fluid heating pass being substantially greater than the number and total internal cross-sectional area of the tubes in the second fluid heating pass to provide a fluid mass flow in the second fluid heating pass greater than the fluid mass flow in the first fluid heating pass, means rigidly uniting tubes of the first and second fluid heating passes, and means rigidly uniting tubes of the third fluid heating pass.

8. In a forced circulation fluid heating unit, upright walls forming a furnace for flow of heating gases, means supplying high temperature heating gases to the furnace, one of the walls having rigidly united first and second fluid heating passes each comprising a multiplicity of upflow tubes arranged in spaced relation and for parallel flow of fluid therethrough, the first and second fluid heating passes being constructed and arranged to provide flow of fluid in the furnace only in an upward direction, means supplying a vaporizable fluid to tubes of the first fluid heating pass, means interconnecting the fluid heating passes to provide serial upflow of fluid successively through tubes of the first fluid heating pass and tubes of the second fluid heating pass, the number and the total internal cross-sectional area of the tubes of the first fluid heating pass being substantially greater than the number and total internal crosssectional area of the tubes of the second fluid heating pass to provide a fluid mass flow in the latter greater than the fluid mass flow in the former, and means rigidly uniting tubes of the first and-second fluid heating passes.

9. In a forced circulation fluid heating unit, upright walls forming a furnace for a flow of heating gases, means supplying high temperature heating gases to the furnace, one of the walls having coextensive rigidly united first and second fluid heating passes of substantially the same length each comprising a multiplicity of upflow tubes arranged in spaced relation and for parallel flow of fluid therethrough, the first and second fluid heating passes being constructed and'arranged to provide flow of fluid in the furnace only in an upward direction, means supplying a vaporizable fluid to tubes of the first fluid heating pass, means interconnecting the fluid heating passes to provide serial upflow of fluid successively through tubes of the first fluid heating pass and tubes of the second fluid heating pass, the number and the total internal cross-sectional area of the tubes of the first fluid heating passes being substantially greater than the number and total internal crosssectional area of the tubes of the second fluid heating pass to provide a fluid mass flow in the latter greater than the fluid mass flow in the former, and means rigidly uniting tubes of the first and second fluid heating passes along substantially their entire lengths.

10. In a forced circulation fluid heating unit, upright walls forming a furnace for flow of heating gases, means supplying high temperature heating gases to the furnace, the walls including rigidly united first and second fluid heating passes each comprising a multiplicity of upflow tubes arranged in spaced relation and for parallel flow of fluid therethrough, the first and second fluid heating passes being constructed and arranged to provide flow of fluid in the furnace only in an upward direction, means supplying a vaporizable fluid to tubes of the first fluid heating pass, means interconnecting the fluid heating passes to provide serial upflowof fluid successively through tubes of the first fluid heating pass and tubes of the second fluid heating pass, the number and the total internal cross-sectional area of the tubes of one of the fluid heating passes being substantially greater than the number and total internal crosssectional area of the tubes of the other fluid heating pass to provide a fluid mass flow in the latter greater than the fluid mass flow in the former, and means rigidly uniting tubes of the first and second fluid heating passes.

11. in a forced circulation fluid heating unit, upright walls forming a furnace for a flow of heating gases, means supplying high temperature heating gases to the furnace, the walls including coextensive rigidly united first and second fluid heating passes of substantially the same length each comprising a multiplicity of upflow tubes arranged in spaced relation and for parallel flow of fluid therethrough, the first and second fluid heating passes being constructed and arranged to provide flow of fluid in the furnace only in an upward direction, means supplying a vaporizable fluid to tubes of the first fluid heating pass, means interconnecting the fluid heating passes to provide serial upflow of fluid successively through tubes of the first fluid heating pass and tubes of the second fluid heating pass, the number and the total internal cross-sectional area of the tubes of the first fluid heating pass being substantially greater than the number and total internal cross-sectional area of the tubes of the second fluid heating pass to provide a fluid mass flow in the latter greater than the fluid mass flow in the former, and means rigidly uniting tubes of the first and second fluid heating passes along substantially their entire lengths. 

1. In a forced circulation fluid heating unit, upright walls forming a furnace for flow of heating gases, means supplying high temperature heating gases to said furnace, one of said walls having a first fluid heating pass comprising a multiplicity of upflow tubes arranged in spaced relation and for parallel flow of fluid therethrough, a second fluid heating pass comprising a multiplicity of upflow tubes interlaced with upflow tubes of the first fluid heating pass and arranged for parallel flow of fluid therethrough, said first and second fluid heating passes being constructed and arranged to provide flow of fluid in the furnace only in an upward direction, means supplying a vaporizable fluid to the fluid heating passes, means interconnecting the fluid heating passes to provide serial flow of fluid therethrough, the number and the total internal cross-sectional area of the tubes of one of the fluid heating passes being substantially greater than the number and total internal cross-sectional area of the tubes of the other fluid heating pass to provide a fluid mass flow in the latter greater than the fluid mass flow in the former, and means rigidly uniting tubes of the first and second fluid heating passes.
 2. In a forced circulation fluid heating unit, upright walls forming a furnace for flow of heating gases, means supplying high temperature heating gases to said furnace, one of said walls having a first fluid heating pass comprising a multiplicity of upflow tubes arranged in spaced relation and for parallel flow of fluid therethrough, a second fluid heating pass of substantially the same length as the first fluid heating pass comprising a multiplicity of upflow tubes interlaced and coextensive with upflow tubes of the first fluid heating pass and arranged for parallel flow of fluid therethrough, said first and second fluid heating passes being constructed and arranged to provide flow of fluid in the furnace only in an upward direction, means for passing a vaporizable fluid to tubes of the first fluid heating pass, means interconnecting said fluid heating passes to provide serial upflow of fluid successively through tubes of the first fluid heating pass and tubes of the second fluid heating pass, the number and the total internal cross-sectional area of the tubes of one of the fluid heating passes being substantially greater than the number and total internal cross-sectional area of the tubes in the other fluid heating pass to provide a fluid mass flow in the latter greater than the fluid mass flow in the former, and metallic web means rigidly uniting tubes of the first and second fluid heating passes.
 3. In a forced circulation fluid heating unit, upright walls forming a furnace for flow of heating gases, means supplying high temperature heating gases to said furnace, one of said walls having a first fluid heating pass comprising a multiplicity of upflow tubes arranged in spaced relation and for parallel flow of fluid therethrough, a second fluid heating pass comprising a multiplicity of upflow tubes interlaced with upflow tubes of the first fluid heating pass and arranged for parallel flow of fluid therethrough, said first and second fluid heating passes being constructed and arranged to provide flow of fluid in the furnace only in an upward direction, means for passing a vaporizable fLuid to tubes of the first fluid heating pass, means interconnecting said fluid heating passes to provide serial upflow of fluid successively through tubes of the first fluid heating pass and tubes of the second fluid heating pass, the number and the total internal cross-sectional area of the tubes in the first fluid heating pass being substantially greater than the number and total internal cross-sectional area of the tubes in the second fluid heating pass to provide a fluid mass flow in the second fluid heating pass greater than the fluid mass flow in the first fluid heating pass, and means rigidly uniting tubes of the first and second fluid heating passes.
 4. In a forced circulation fluid heating unit, upright walls forming a furnace for flow of heating gases, means supplying high temperature heating gases to said furnace, one of said walls having a first fluid heating pass comprising a multiplicity of upflow tubes arranged in spaced relation and for parallel flow of fluid therethrough, a second fluid heating pass of substantially the same length as the first fluid heating pass comprising a multiplicity of upflow tubes interlaced and coextensive with upflow tubes of the first fluid heating pass and arranged for parallel flow of fluid therethrough, said first and second fluid heating passes being constructed and arranged to provide flow of fluid in the furnace only in an upward direction, means for passing a vaporizable fluid to tubes of the first fluid heating pass, means interconnecting said fluid heating passes to provide serial upflow of fluid successively through tubes of the first fluid heating pass and tubes of the second fluid heating pass, the number and the total internal cross-sectional area of the tubes in the first fluid heating pass being substantially greater than the number and total internal cross-sectional area of the tubes in the second fluid heating to provide a fluid mass flow in the second fluid heating pass greater than the fluid mass flow in the first fluid heating pass, and metallic web means rigidly uniting tubes of the first and second fluid heating passes along substantially their entire lengths.
 5. In a forced circulation fluid heating unit, upright walls forming a furnace for flow of heating gases, means supplying high temperature heating gases to said furnace, one of said walls having a first fluid heating pass comprising a multiplicity of upflow tubes arranged in spaced relation and for parallel flow of fluid therethrough, a second fluid heating pass comprising a multiplicity of upflow tubes interlaced with upflow tubes of the first fluid heating pass and arranged for parallel flow of fluid therethrough, a third fluid heating pass comprising a multiplicity of upflow tubes above the tubes of the first and second fluid heating passes, means for passing a vaporizable fluid to tubes of the first fluid heating pass, means interconnecting said fluid heating passes to provide serial upflow of fluid successively through tubes of the first fluid heating pass, tubes of the second fluid heating pass and tubes of the third fluid heating pass, the number and the total internal cross-sectional area of the tubes in the first fluid heating pass being substantially greater than the number and total internal cross-sectional area of the tubes in the second fluid heating pass to provide a fluid mass flow in the second fluid heating pass greater than the fluid mass flow in the first fluid heating pass, means rigidly uniting tubes of the first and second fluid heating passes, and means rigidly uniting tubes of the third fluid heating pass.
 6. In a forced circulation fluid heating unit, upright walls forming a furnace for flow of heating gases, means supplying high temperature heating gases to said furnace, one of said walls having a first fluid heating pass comprising a multiplicity of upflow tubes arranged in spaced relation and for parallel flow of fluid therethrough, a second fluid heating pass of substantially the same length as the first fluid heatiNg pass comprising a multiplicity of upflow tubes interlaced and coextensive with upflow tubes of the first fluid heating pass and arranged for parallel flow of fluid therethrough, a third fluid heating pass comprising a multiplicity of upflow tubes above the tubes of the first and second fluid heating passes, means for passing a vaporizable fluid to tubes of the first fluid heating pass, means interconnecting said fluid heating passes to provide serial upflow of fluid successively through tubes of the first fluid heating pass, tubes of the second fluid heating pass and tubes of the third fluid heating pass, the number and the total internal cross-sectional area of the tubes in the first fluid heating pass being substantially greater than the number and total internal cross-sectional area of the tubes in the second fluid heating pass to provide a fluid mass flow in the second fluid heating pass greater than the fluid mass flow in the first fluid heating pass, metallic web means rigidly uniting tubes of the first and second fluid heating passes along substantially their entire lengths, and metallic web means rigidly uniting tubes of the third fluid heating pass along substantially their entire lengths.
 7. In a forced circulation fluid heating unit, upright walls forming a furnace for flow of heating gases, means supplying high temperature heating gases to said furnace, one of said walls having a first fluid heating pass comprising a multiplicity of upflow tubes arranged in spaced relation and for parallel flow of fluid therethrough, a second fluid heating pass comprising a multiplicity of upflow tubes interlaced with upflow tubes of the first fluid heating pass and arranged for parallel flow of fluid therethrough, a third fluid heating pass comprising a multiplicity of upflow tubes above the tubes of the first and second fluid heating passes, means for passing a vaporizable fluid to tubes of the first fluid heating pass, means interconnecting said fluid heating passes to provide serial upflow of fluid successively through tubes of the first fluid heating pass, tubes of the second fluid heating pass and tubes of the third fluid heating pass, tubes of the third fluid heating pass having lower portions interlaced with upper portions of tubes of the second fluid heating pass and aligned with tubes of the first fluid heating pass, the number and the total internal cross-sectional area of the tubes in the first fluid heating pass being substantially greater than the number and total internal cross-sectional area of the tubes in the second fluid heating pass to provide a fluid mass flow in the second fluid heating pass greater than the fluid mass flow in the first fluid heating pass, means rigidly uniting tubes of the first and second fluid heating passes, and means rigidly uniting tubes of the third fluid heating pass.
 8. In a forced circulation fluid heating unit, upright walls forming a furnace for flow of heating gases, means supplying high temperature heating gases to the furnace, one of the walls having rigidly united first and second fluid heating passes each comprising a multiplicity of upflow tubes arranged in spaced relation and for parallel flow of fluid therethrough, the first and second fluid heating passes being constructed and arranged to provide flow of fluid in the furnace only in an upward direction, means supplying a vaporizable fluid to tubes of the first fluid heating pass, means interconnecting the fluid heating passes to provide serial upflow of fluid successively through tubes of the first fluid heating pass and tubes of the second fluid heating pass, the number and the total internal cross-sectional area of the tubes of the first fluid heating pass being substantially greater than the number and total internal cross-sectional area of the tubes of the second fluid heating pass to provide a fluid mass flow in the latter greater than the fluid mass flow in the former, and means rigidly uniting tubes of the first and second fluid heating passes.
 9. In a forced circulation fluid heating unit, upright walls forming a furnace for a flow of heating gases, means supplying high temperature heating gases to the furnace, one of the walls having coextensive rigidly united first and second fluid heating passes of substantially the same length each comprising a multiplicity of upflow tubes arranged in spaced relation and for parallel flow of fluid therethrough, the first and second fluid heating passes being constructed and arranged to provide flow of fluid in the furnace only in an upward direction, means supplying a vaporizable fluid to tubes of the first fluid heating pass, means interconnecting the fluid heating passes to provide serial upflow of fluid successively through tubes of the first fluid heating pass and tubes of the second fluid heating pass, the number and the total internal cross-sectional area of the tubes of the first fluid heating passes being substantially greater than the number and total internal cross-sectional area of the tubes of the second fluid heating pass to provide a fluid mass flow in the latter greater than the fluid mass flow in the former, and means rigidly uniting tubes of the first and second fluid heating passes along substantially their entire lengths.
 10. In a forced circulation fluid heating unit, upright walls forming a furnace for flow of heating gases, means supplying high temperature heating gases to the furnace, the walls including rigidly united first and second fluid heating passes each comprising a multiplicity of upflow tubes arranged in spaced relation and for parallel flow of fluid therethrough, the first and second fluid heating passes being constructed and arranged to provide flow of fluid in the furnace only in an upward direction, means supplying a vaporizable fluid to tubes of the first fluid heating pass, means interconnecting the fluid heating passes to provide serial upflow of fluid successively through tubes of the first fluid heating pass and tubes of the second fluid heating pass, the number and the total internal cross-sectional area of the tubes of one of the fluid heating passes being substantially greater than the number and total internal cross-sectional area of the tubes of the other fluid heating pass to provide a fluid mass flow in the latter greater than the fluid mass flow in the former, and means rigidly uniting tubes of the first and second fluid heating passes.
 11. In a forced circulation fluid heating unit, upright walls forming a furnace for a flow of heating gases, means supplying high temperature heating gases to the furnace, the walls including coextensive rigidly united first and second fluid heating passes of substantially the same length each comprising a multiplicity of upflow tubes arranged in spaced relation and for parallel flow of fluid therethrough, the first and second fluid heating passes being constructed and arranged to provide flow of fluid in the furnace only in an upward direction, means supplying a vaporizable fluid to tubes of the first fluid heating pass, means interconnecting the fluid heating passes to provide serial upflow of fluid successively through tubes of the first fluid heating pass and tubes of the second fluid heating pass, the number and the total internal cross-sectional area of the tubes of the first fluid heating pass being substantially greater than the number and total internal cross-sectional area of the tubes of the second fluid heating pass to provide a fluid mass flow in the latter greater than the fluid mass flow in the former, and means rigidly uniting tubes of the first and second fluid heating passes along substantially their entire lengths. 